The present invention relates to the surface modification of shaped objects composed, in whole or in part, of hydrophobic polymers.
Polymers are used widely throughout the world to make a variety of products which include blown and cast films, extruded sheets, injection molded articles, foams, blow molded articles, extruded pipe, monofilaments, and nonwoven webs. Some of such polymers, such as polyolefins, are naturally hydrophobic, and for many uses this property is either a positive attribute or at least not a disadvantage.
There are a number of uses for hydrophobic polymers, however, where their hydrophobic nature either limits their usefulness or requires some effort to modify the surface characteristics of the shaped articles made therefrom. By way of example, polyolefins are used to manufacture nonwoven webs which are employed in the construction of such disposable absorbent articles as diapers, feminine care products, incontinence products, and the like. Frequently, such nonwoven webs need to be wettable. Wettability can be obtained by spraying or coating the web with a surfactant solution during or after its formation and then drying the web. However, surfactant which remains on the web is removed upon exposure of the web to aqueous media. Alternatively, a surfactant can be included in the polymer which is to be melt-processed, as disclosed in U.S. Pat. Nos. 3,973,068 and 4,070,218 to R. E. Weber. In that case, however, the surfactant must be forced to the surface of the fibers from which the web is formed. This typically is done by heating the web on a series of steam-heated rolls or "hot cans". This process, called "blooming", is expensive and still has the disadvantage of ready removal of the surfactant by aqueous media. Moreover, the surfactant has a tendency to migrate back into the fiber which adversely affects shelf life, particularly at high storage temperatures. In addition, it is not possible to incorporate in the polymer levels of surfactant much above 1 percent by weight because of severe processability problems; surfactant levels at the surface appear to be limited to a maximum of about 0.33 percent by weight. Most importantly, the blooming process results in web shrinkage in the cross-machine direction and a significant loss in web tensile strength.
Other methods of imparting wettability to, or otherwise affecting the surface characteristics of, shaped articles made from polyolefins and other hydrophobic polymers are known. For example, an alkoxylated alkyl phenol in combination with a mixed mono-, di-, and/or triglyceride and/or a polyoxyalkylene fatty acid ester have been incorporated in a polyolefin resin and the resulting composition utilized to form wettable olefin polymer fibers (U.S. Pat. No. 4,578,414 to Sawyer and Knight). As another example, a substrate has been contacted with an aqueous mixture containing a water-soluble vinyl monomer and a hydrophobic vinyl monomer. Polymerization of the water-soluble vinyl monomer then was initiated by a polymerization initiator, thereby forming a vinyl polymer on the surface of the polymer substrate (U.S. Pat. No. 4,672,005 to Dyer). As a further example, the surface of a polymer material has been modified by means of a block copolymer. The block copolymer consists of a hydrophilic polymer portion formed from a vinyl monomer and a polymer portion which is compatible with the polymer material, also formed from a vinyl monomer (U.S. Pat. No. 4,698,388 to Ohmura et al.).
Poly(vinyl alcohol) has interesting physical and chemical properties which are associated with its affinity for water. Crosslinked and noncrosslinked hydrogels of the polymer are used in numerous devices such as contact lenses (U.S. Pat. No. 4,695,037), composite glass (U.S. Pat. No. 5,367,015), and synthetic papers (U.S. Pat. Nos. 3,560,318; 4,002,796; 4,152,317; and 4,510.185). Coatings of poly(vinyl alcohol) are described in the patent literature as being formed by deposition of aqueous emulsions of the polymer on hydrophobic substrates which most typically are fibrils of high density polyethylene for use in synthetic paper (U.S. Pat. Nos. 3,560,318; 4,002,796; 4,152,317; and 4,510.185). In addition, the surface modification of polyamides has been described, wherein poly(vinyl alcohol) has been partially esterified with polycarboxylic acids. The partially esterified material is bound to the polyamide surface via free carboxylate functionalities and reactive groups on the polyamide surface (U.S. Pat. No. 3,050,418). A composite material formed from poly(vinyl alcohol), a modified starch, and water-soluble cellulose has found utility as an aqueous-based size for cotton and cotton/polyester yarns (U.S. Pat. No. 5,420,180).
The fluorination of vinyl monomers and the polymerization of these monomers to form fluoropolymers has been described in U.S. Pat. No. 2,436,144 to Howk and Jacobson. The patent describes a method for preparing vinyl trifluoroacetate. The patent also describes the polymerization of the monomer to form a tough, nonflammable, and thermoformable polymer which they named poly(vinyl trifluoroacetate).
Poly(vinyl alcohol) cannot be made by polymerization of vinyl alcohol. Vinyl alcohol is the enol of acetaldehyde, and as such exists in equilibrium with its aldehydic tautomer. Poly(vinyl alcohol) typically is synthesized by the hydrolysis or solvolysis of other vinyl polymers having pendant groups which will yield the alcohol and an appropriate leaving group. Examples of polymers used in the preparation of poly(vinyl alcohol) include, by way of illustration only, poly(vinyl acetate), poly(vinyl t-butyl ether), and poly(vinyl acetal). Due to the commercial importance of poly(vinyl alcohol) as a yarn sizing and stabilizer for aqueous emulsions and dispersions, the nonaqueous solution chemistry of the polymer and its esters has been largely ignored. Aside from mentioning that poly(vinyl trifluoroacetate) was readily soluble in N,N-di-methylformamide, U.S. Pat. No. 2,436,144 does not explore the properties of nonaqueous poly(vinyl trifluoroacetate) solutions.
There is, therefore, a need for methodology which will permit the uniform coating of hydrophobic polymer surfaces with a poly(vinyl alcohol) in order to render such surfaces hydrophilic. There also is a need for methodology which will permit the uniform coating of hydrophobic polymer surfaces with a hydrophobic vinyl polymer in order to alter the surface characteristics of the hydrophobic polymer.